This invention relates to color calibration for digital color reproduction apparatus such as, for example a color fax or printer.
Color reproduction apparatus renders an image from color signals representing a color image description. The input to such a color reproduction apparatus is an array of color signals (sets of input code values) communicating the desired colors of a corresponding array of picture elements in the image to be rendered.
The reproduction apparatus causes cyan, magenta and yellow, and sometimes black, dyes to be laid down on a receiving medium in response to color signals (its inherent drive code values) applied to its dye marking or producing devices.
A goal is to make the renderings match an original intent as nearly as possible. In order to do so, a transform is employed which maps the received input code values, normally in RGB space, to the reproduction apparatus"" inherent drive code values, so that the system faithfully reproduces the color image original. Often the transform is implemented by employing a set of three or four one-dimensional calibration lookup tables. The process of deriving such a transform is called system color calibration.
Such apparatus requires re-calibration from time to time due to changes in the apparatus"" behavior, as might be affected by the state of processor chemicals if any are used, color marking system drifts, and changes in receiver characteristics. Differences in receiver characteristics might be caused by different recipes, variations in receivers from one coating event to another, and different surface finishes (i.e., glossy verses matte finishes) as are commonly encountered using photographic receivers.
Photographic papers are sold by manufacturer and type. For example Eastman Kodak Company sells among others, Edge 7(trademark) optical printing paper and Type 2976(trademark) digital paper Eastman Kodak Company and the other paper manufacturers coat their papers in batches called xe2x80x9ccoating events.xe2x80x9d For each coating event, fresh emulsions are mixed and tested for conformance to aim characteristics. Notwithstanding all efforts to make each batch identical, there will be some very small batch-to-batch variability from the aim characteristics for that paper type. When paper is packaged for sale, it is labeled by manufacturer and type with identifiers which indicate the coating event from which that particular paper was cut.
Generally, a lab will purchase large quantities of paper at one time to ensure that its stock came from a single coating event. However, in many instances, papers of the same type, but from different coating events will be in use at the same time. Papers or other receivers which are of a certain type but which may come from different coating events are referred to herein as members of a group.
Calibration of a printer typically accounts for every parameter that can cause density variations of a print. This includes writing system, receiver, chemistry etc. Accordingly, different receiver types must be separately calibrated. However, if the variations between members of a group are small, then a lab may be satisfied calibrating on only one member of that group, regardless of coating event. For best results, however, each instance of a coating event should be calibrated separately every day.
It is an object of the present invention to provide a system to periodically calibrate only one member of a group of receivers of a certain type (say, each day) and to apply a correction for other members of the group based on their relative differences from the one member. The one member to be calibrated periodically is designated the xe2x80x9ckeyxe2x80x9d member. Whenever a new member of the group comes into the lab, it, along with the key member is calibrated at more or less the same time. The ratios of the density outputs over the full exposure range are calculated and saved in what is herein called a xe2x80x9cdelta filexe2x80x9d for the new member. Then, at each reproduction apparatus calibration, the density outputs for the key member are determined and the ratios from the delta files are applied to predict the corresponding density outputs of the other group members, as if they were being calibrated at the same time. By this process, only one calibration (the calibration of the key member) is done on each group and each reproduction apparatus. If more than one group is being used, then a separate calibration is effected for the key member of each group.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below.